Page 1
NASA CASE NO.
PRINT FIG. ___
NOTICE
The invention disclosed in this document resulted from research in aeronautical
and space activities performed under programs of the National Aeronautics and Space
Administration. The invention is owned by NASA and is, therefore, available for
licensing in accordance with the patent licensing regulations applicable to U.S.
Government-owned inventions (37 CFR 404.1 at secl.).
To encourage commercial utilization of NASA-owned inventions, it is NASA
policy to grant licenses to commercial concerns. Although NASA encourages
nonexclusive licensing to promote competition and achieve the widest possible
utilization, NASA will provide the necessary incentive to the licensee to achieve early
practical application of the invention.
Address informational requests for this invention to the Technology
Applications Team, NASA Langley Research Center, Code 200, Hampton, Virginia
23681-0001.
Address applications for license to the Patent Counsel Office, Mail Stop 212,
3 Langley Boulevard, Hampton, Virginia 23681-0001.
Serial No.: 08/413,301
Filed: 03/30/95
(NASA-Case-LAR-14898-1) COMPOSITE
SANDWICH STRUCTURE AND METHOD FOR
MAKING SAME Patent Application
(NASA. Langley Research Center}
32 p
Unclas
G3/24 0049666
https://ntrs.nasa.gov/search.jsp?R=19950020344 2020-08-02T23:58:31+00:00Z
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LAR- 1 4898- 1 AWARDS ABSTRACT
COMPOSITE SANDWICH STRUCTURE AND METHOD FOR MAKING SAME
The present invention relates in general to light weight composite sandwich structures, and specifically to improved fabrication of thermally stable optical bench sandwich structures used as substructure for and critical to the performance of highly precise instruments such as space telescopes, interferometric optics, and reflector panels.
According to the invention, a core comprising multi-ply laminate ribs separated by voids is made as an integral unit in one single curing step. Tooling blocks corresponding to the voids are first wrapped by strips of prepreg layup equal to one half of each rib laminate so a continuous wall of prepreg material is formed around the tooling blocks. The wrapped tooling blocks are next pressed together laterally, like tiles, so adjoining walls from two tooling blocks are joined. The assembly is then cured by conventional methods, and afterwards the tooling blocks are removed so voids are formed. The ribs can be provided with integral tabs forming bonding areas for face sheets, and face sheets may be co-cured with the core ribs.
The new core design can be built with quasi-isotropic properties and zero coefficient of thermal expansion. Cores, and corresponding sandwich structures according to the invention have exceptional mechanical strength and integrity. The manufacturing process requires no cutting and fitting of ribs, nor any secondary bonding of rib junctions, so the manufacturing process is very cost effective, highly repeatable, and does not require highly skilled workers.
Inventor: Charles J. Magurany Home address: 29600 Island View Drive #204
Rancho Palos Verdes, CA 90275
Employer: SS#: Lockheed Engineering and Space Company
Evaluator: David E. Bowles
Serial No.: 08/413,301 Filed: March 30, 1995
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COMPOSITE SANDWICH STRUCTUREAND
METHOD FORMAKING SAME
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Origin nf the Invention
The invention described herein was made in the performance of work
under a NASA Contract and is subject to the provisions of Section 305 of the
National Aeronautics and Space Act of 1958, as amended, Public Law
85-568 (72 Stat. 435; 42 USC 2457).
Background of the Invention
Field of the Invi_ntion
15
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This invention relates in general to light weight composite
sandwich structures, and specifically to improved fabrication of thermally
stable optical bench sandwich structures, used as substructure for highly
precise instruments such as space telescopes, interferometric optics, and
reflector panels.
Description of the Related Art
25
There are two types of construction methods currently used to
fabricate composite optical bench sandwich structures: honeycomb core
and discrete (or eggcrate) rib-core. For space applications the discrete
rib-core design has several advantages over honeycomb cores, as is well
known to those skilled in the art.
The prior art discrete rib-core approach relies upon a secondary
assembly and adhesive joint bonding of pre-cured and precision machined
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graphite/epoxy laminate ribs. The ribs are positioned to form a grid/core
structure which is then bonded to face skins. The rib spacing and
geometry is customized to meet the required loading conditions and
fitting interfaces.
5 The fabrication of a discrete rib-core starts with cutting of
graphite/epoxy prepreg tape from a roll into single plys of predetermined
lengths and orientation angles. For thermally stable space applications
this usually requires ultra-high modulus (UHM) fiber/epoxy plys oriented
and stacked into a quasi-isotropic (QI) layup. A composite using QI layup
10 is intended to approximate a structure having equal mechanical and
physical properties in all directions within the plane of the laminate.
For precision composite optical benches, zero (or near zero)
coefficient of tl_ermal expansion (CTE) and zero moisture strain are
desired design properties. It has been calculated that a QI layup using
15 UHM P75 fiber/934 epoxy, at approximately 51% fiber by volume, will
, achieve a near zero in-plane CTE. A suitable QI layup would comprise 8
plys having fiber orientations: [0,45,90,-45,-45,90,45,0].
The dimensions of the plys used to layup the panel are calculated
to ensure that sufficient material will be cured to meet the requirements
20 for the quantity of ribs that will be needed to assemble the core
structure. After layup, the panel is processed into a cured flat laminate
by conventional autoclave methods.
After cure, the panel must be accurately and precisely machined
into strips having the required dimensions for the final core structure.
25 After the strips are cut, vertical joint location slots of prescribed length,
width and location must be accurately and precisely machined into each
of the ribs. The number of slots machined depends upon the dimensions
of the final core structure.
Much of the design strength and structural integrity of the rib-core
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structure will depend upon the joint (or slot-to-slot) fastening or bonding
method used. There are three basic graphiteto-graphite joint types that
have been used for optical benches: unclipped butt joints, clipped butt
joints, and mortise and tenon joints.
After the ribs have been positioned into the overall core structure,
they must be structurally fixed into place, at the joint interfaces, through
adhesive bonding. Before adhesive is applied, the ribs of the assembly,
still free to move depending upon the tightness of the joint slot overlaps,
must be uniformly held in place in a manner that enables correct overall
rib alignment and orthogonality. Once alignment is achieved, a bead of
an appropriate adhesive is applied to the four interfaces of each joint, i.e.
4 beads per joint. The adhesive is cured by oven or at room
temperature, depending on the adhesive used. Afterwards, joint
reinforcing hardware may be installed to improve strength characteristics.
The necessary, accurate and precise machining of the cured
laminate to produce the separate ribs and the machining to produce
multiple slots for each rib, is a costly, time consuming activity which
requires skilled labor. Uncorrected changes or wear in the cutting
equipment over a period of time will cause variation in slot sizes and rib
dimensions which can affect variations in the thermal/mechanical
properties of the core. If proper tolerances are not met, the parts must
either be reworked or completely remanufactured.
The consistency of the process is dependent upon the availability
of skilled/experienced labor. The repetitive hand-application of adhesive
to each and every joint is tedious and even the most skilled technician is
prone to produce work having variations. Correct alignment of the ribs is
critical to the core structure performance and must be maintained while
the adhesive is applied and excess removed. Bonding tool jigs, required
to maintain rib alignment, interfere with the technicians' access to the
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joint bonding locations, making this process especially cumbersome.
Given the high degree of artistry required, scale-up of this process is
difficult, requiring longer lead-times and, likely, rework.
Ideally, the core should have the same QI properties as the
5 laminate face sheets. This would ensure that the optical bench sandwich
structure will respond to mechanical and thermal loads in a uniform
manner (i.e., zero CTE throughout the planes of the ribs of the core, as
well as within the planes of the bonded face sheets). Also, an efficient
design/process will enable localized loads to be distributed over the entire
10 structure, thus reducing stresses and cause for failure at any one point.
In the prior art process, discontinuities in the rib laminates, caused
by slotting, disrupt the normal load carrying capabilities of the ribs. The
loads can be in'tension, compression, or shear. The adhesive, without
fiber reinforcement, must act to transfer and distribute these loads
15 among the ribs at the joint locations. The proper design use of adhesives
, is to carry mechanical loads in shear as adhesive interfacial tension
properties are typically very low. The rib-core process/design does not
exclude the joint adhesive from carrying significant interfacial tension
loads. Also, the loads are carried along a thin bondline (four at each
20 joint). This limits the ability of the joint to transfer/distribute loads to the
rib structure, causing higher stresses to occur at the weaker
joint/adhesive interfaces. In accordance with the rib-core design/process,
load must be carried/transferred at a finite number of joint locations. The
fewer the number of the joints in the core structure, the higher the
25 stresses at each joint, and the weaker the resulting core structure. To
compensate for the inherent weakness in the rib-core design/process,
secondary "butt joint clips" and other methods have been applied to
strengthen the adhesive joint interfaces. These methods lead to
additional weight and significant time and cost increases in labor and
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materials. In addition to these drawbacks, adhesives commonly exhibit
"creep" under constant loading, again contributing to possible structural
deformations.
Aside from the mechanical strength mismatch between the
adhesive and ribs, the corresponding CTE mismatch also has adverse
effects on the response to thermal loads of the sandwich structure. The
CTE of adhesives is typically greater than 25 microstrain per degree F,
while that of the rib laminates is designed to be zero. Because of this
mismatch, room temperature cure adhesives are commonly used to avoid
thermal cure stresses at the joint locations. This does not, however,
prevent the thermal stresses that will occur in certain space applications
in which a structure may be exposed to temperature extremes anywhere
from 250°F to -'250°F. The resulting thermal stresses at the core joints
can adversely affect the performance of the overall structure in three
primary ways: (a) the through-the-thickness CTE of the sandwich
structure can deviate, misaligning sensitive optics out of tolerance; (b)
thermal cycling at the temperature extremes can eventually cause
adhesive failure; and (c) uneven thermal loads at the adhesive joints can
cause significant stress gradients within the core structure, causing non-
uniform deformation in the structure.
The additional launch costs, due to increased weight, using a
process that relies on adhesive, butt joint clips, etc., at the joints can be
substantial for a large structure.
U.S. Patent No. 5,342,679 to _ describes a method for
making integrally formed discrete rib-cores that does not require cutting
slots or gluing the ribs together. A_c, hi at al. discloses first wrapping
fibers horizontally over pegs to form the desired rib structure, and then
inserting mandrels wrapped with fibers oriented vertically into the spaces
between the ribs. Matrix material is then poured into the spaces
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between the mandrels and cured, and the fibers are cut off outside the
desired rib structure. Both vertical and horizontal fibers are included in
the final rib-core, however, a QI layup is not possible. The fiber content
is also limited in the horizontal direction by the wrapping process to a
5 very low percentage of the finished laminate. The fabrication method
described by _ is accordingly not applicable to highly stable
optical bench sandwich structures.
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Summary of the Invention
Accordingly, it is an object of the present invention to provide
integrated discrete rib-cores suitable for optical bench sandwich
structures, free'from adhesives and cuts in the ribs.
It is a further object of the invention to provide a method for
making integrated discrete rib-cores suitable for optical bench sandwich
structures without need for cuts in the ribs and gluing of ribs.
It is another object of the present invention to provide a low cost
method for making high quality integrated cores of both discrete rib
design and honeycomb design.
It is a still further object of the present invention to provide a
method for making integrated core and face sheet sandwich structures
with quasi-isotropic properties and near zero coefficient of thermal
expansion (CTE).
It is a still another object of the present invention to provide a
simple and reliable method for making high strength light weight
composite cores and sandwich structures without need for highly skilled
workers.
These and other objects are achieved by a composite sandwich
structure comprising a plurality of interconnecting ribs separated by
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PATENT APPLICATION
voids, each of the voids being surrounded by a substantially continuous
wall of composite fiber reinforced material, and each of said ribs
consisting of composite fiber reinforced material from the walls of two
adjoining voids.
These and other objects are further achieved by a method for
making such a sandwich structure comprising the steps of providing a
plurality of tooling blocks having cross sections fitting together like tiles,
providing prepreg laminate material, applying prepreg material around the
sides of each of the tooling blocks, arranging the tooling blocks with
prepreg laminate material applied in lateral contact to form a tooling block
assembly, curing the prepreg material in the tooling block assembly to
form a composite material, and removing the tooling blocks from the
cured composite material.
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Brief Dascription of tha Drawings
The present invention and the objects achieved by it will be
understood from the description herein, with reference to the
accompanying drawings, in which:
FIG. 1 is a top view of an eggcrate rib-core according to the
invention.
FIG. 2 is an enlarged top view of a point where ribs meet in the
eggcrate rib-core of FIG. 1.
FIG. 3 is an enlarged vertical section through a rib in the eggcrate
rib-core of FIG. 1 as indicated in FIG. 2.
FIG. 4 is an enlarged vertical section diagonally through a corner
between ribs in the eggcrate rib-core of FIG. 1 as indicated in FIG. 2.
FIG. 5 is a top view of an assembly of parts for making an
eggcrate rib-core according to the invention.
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FIG. 6 is a top view of a tooling block wrapped with prepreg
material with a butt joint.
FIG. 7 is an enlarged partial top view of an alternative end-to-end
joint for a strip of multi-layer prepreg material wrapped around a tooling
5 block.
FIG. 8 is a top view of a jig for use with the assembly shown in
FIG. 5 with one tooling block in place.
FIG. 9 is a top view of honeycomb cores according to the
invention.
10 FIG. 10 is a top view of honeycomb cores according to the
invention.
FIG. 11 is
material forming
FIG. 12 is
material forming
a lateral view of a tooling block wrapped by prepreg
tabs on the top of the tooling block.
a top view of a tooling block wrapped by prepreg
15 tabs on the top of the tooling block.
• FIG. 13 is a lateral view of an assembly as illustrated in FIG. 5 with
tooling blocks wrapped with prepreg material as shown in FIGS. 11 and
12.
FIG. 14 is a lateral view of a tooling block for an eggcrate rib-core
20 wrapped with two strips of prepreg material crossing at the top and
draped over opposed sides of the tooling block.
FIG. 15 is a lateral view of an assembly as shown in FIG. 5 with
tooling blocks wrapped with prepreg material as shown in FIG. 14, plus a
top caul plate.
25 FIG. 16 is a schematic top view of an assembly of tooling blocks
wrapped with multi layer prepreg material as shown in FIG. 14, with
preferred orientations of the prepreg material indicated.
FIG. 17 is a perspective view of a tooling block assembly as
shown in FIG. 16 indicating details of preferred orientations of prepreg
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-9- PATENT APPLICATION
Detailed Description nf the Preferred Embodiments
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FIG. 1 shows a top view of a discrete ("eggcrate") rib-core 10
according to the present invention. The core 10 looks like a conventional
eggcrate structure with nine square voids. For purposes of explanation
only, three square voids 14, 14' and 14" are described herein. The voids
14, 14', and 14" are separated by ribs 12 and 12', but the ribs 12 and 12'
do not intersect where they meet at corner 13. Instead, each void 14, 14',
and 14" is enclosed by continuous laminate walls 15, 16, and 17 of
composite material. The adjoining wall sections of the continuous walls 15,
16 and 17 are co-cured to form a rigid assembly of composite ribs 12 and
12'.
Details of the construction of a core 10 according to the present
invention are shown in FIGS. 2 - 4. FIG. 2 is an enlarged top view of one
corner 13 in the rib-core 10. FIGS. 3 - 4 are, respectively, an enlarged
vertical section through a rib 12, and a vertical section diagonally through
the corner 13, as indicated in FIG. 2. The internal rib 12 is formed by
adjoining wall sections 15 and 16. The adjacent internal rib 12' is formed
by adjoining wall sections 16 and 17. The two ribs 12, 12' meet at the
corner 13, but the laminate walls 15, 16, 17 in the two ribs 12, 12' do not
intersect. The laminate walls 15, 16, 17 are instead bent sharply around
the corners of voids 14, 14', 14" so they barely touch each other
tangentially at the center of corner 13, as indicated in FIG. 4. The space
between corners of the laminate walls 15, 16, 17 will be filled with matrix
material when the laminate walls 15, 16, 17 are cured to form the ribs 12,
12'. Each of two adjoining laminate walls (e.g. 15, 16 or 16, 17) contribute
half of the total laminate in each internal rib (e.g. 12, 12'), as indicated in
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FIG. 3.
rib 12.
-10- PATENT APPLICATION
After curing the two halves form one single, integrated composite
Around the periphery of the core 10 the voids 14, 14' do not adjoin
" any other voids, so there will only be one single laminate wall (e.g. 15, 17).
5 Extra strips of laminate material can, however, be added along the outer
periphery to contribute the missing half of the laminate in these ribs, as will
be described below.
The composite core 10 may be designed from various fiber/resin
types, layups (including QI), and thicknesses to provide the required
10 mechanical and/or physical properties in the core 10. Preferably, the only
materials used in the core 10 are the design-specified fiber and resin. No
adhesives or other add-on material are used.
The dimensions of the core 10 may be extended in either or both
directions by simply including more composite unit modules. The functional
15 use and application of the core 10 is identical to those presently used in the
art.
FIG. 5 is a diagram showing a preferred method for making an
eggcrate rib-core 10 according to the present invention. Nine tooling blocks
(for brevity the discussion will be limited to three tooling blocks 40, 40',
20 40") with square cross sections, are each wrapped with strips 50 of
prepreg material, so a continuous wall of prepreg material is formed around
each tooling block 40, 40', 40" The tooling blocks 40, 40', 40" are
slightly higher than the desired height of the core ribs 12, and their vertical
corner edges are slightly rounded. The edge radius is determined by the
25 drape characteristics for the specific fiber/resin combination used in the
process, and may be as large as 1/16 inch.
The prepreg material used in the strips 50 is the same as that used
by prior art. It may be unidirectional prepreg tape, or woven fabric, or tow,
depending on the properties required. In all cases a sufficient degree of
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resin tack is required to ensure that the pre-cured laminate's fiber
orientation and alignment will not shift during handling in the processing
prior to cure.
In a conventional quasi-isotropic eggcrate rib-core 10, the prepreg
5 material for the ribs are cut from a flat laminate panel, which is laid up ply
by ply to form a quasi-isotropic laminate after debulking (using a hot iron or
vacuum bag) and curing. A typical layup for the prior art would consist of
8 plys oriented at angles [0, 45, 90, -45, -45, 90, 45, 0], but other ply
orientations may be used for different combinations of fiber and matrix
10 material. In either case it is preferable to avoid having plys with 90 degree
orientation difference in direct contact, because this may cause high local
stresses in the cured material during thermal cycling. Ideally the layup
should be symrinetrical about the middle.
For a core 10 having 8 ply quasiisotropic ribs 12, each prepreg strip
15 50 should, however, be only four ply thick, because the finished ribs 12 will
consist of two co-cured laminate halves. A pre-cured 4-ply panel is evenly
cut into strips 50 with width in slight excess of the final core height
required. Strips 50 are then wrapped around the perimeters of the tooling
blocks 40, 40', 40" with ends meeting at a butt joint 52 at one side of each
20 tooling block as shown in FIG. 6.
FIG. 7 is a diagram showing an alternate method for joining the ends
of the prepreg strips 50 wrapped around each tooling block 40, 40', 40".
Generally, a plurality of ply drop-offs 501, 502, 503, 504 are created when
laying up the prepreg panel. When cut into strips and wrapped around the
25 blocks, the ply dropoffs 501, 502, 503, 504 from one end will exactly
overlap the other end, leaving no apparent seam. This approach is
structurally better for load transfer.
If debulking is required to make the laminate strips 50 conform
closely to the perimeter of the tooling blocks 40, 40', 40", shrink tape is
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wrapped around the prepreg strips 50, heat is applied, and the shrink tape
is removed before further processing. Additional prepreg strips 50' are laid
along the outer edges of the assembly of wrapped tooling blocks 40, 40',
40" to provide outer halves of the peripheral ribs.
To facilitate consolidation of the prepreg strips 50, 50' between the
tooling blocks 40 and along the sides, hand pressure is first applied to force
each of the four corner tooling block 40 and the four side tooling blocks 40'
toward the center tooling block 40". This is followed by application of
shrink tape along the outside perimeter of the tooling block assembly and
heating of the shrink tape.
Four caul plates 45 are placed along the edges, and the resulting
assembly is ready to be prepared for conventional autoclave cure
processing, using similar materials and procedures as in the prior art.
During processing, the tooling blocks 40, 40', 40", will conduct cure
temperature heat uniformly to all parts of the laminate 50, 50'. At the same
time, under autoclave pressure, the corner blocks 40 and side blocks 40'
move toward the center block 40" and apply uniform consolidating pressure
to all walls of the laminate.
In order to ensure that all ribs 12 are parallel and of equal thickness,
it is important to maintain symmetry between the individual tooling blocks
40, 40', 40" in the tooling block assembly during curing under autoclave
pressure. Such symmetry can be assured by guiding the individual tooling
blocks 40, 40', 40" in a curing jig. FIG. 8 is a top view of a curing jig 30
for a nine cell eggcrate rib-core, with one tooling block 40 shown in
position. The curing jig 30 comprises a tool base 32 of metal, or any
alternate material capable of withstanding process temperatures while
maintaining rigidity and flatness. Within the tool base 32 are symmetrically
oriented machined slots 34, 36 and holes 38, designed to mate with dowels
or slide-tabs at the bottom of each tooling blocks 40. The ends of each of
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the tool base 32 slots 34, 36 are rounded to facilitate machining.
For a nine cell rib-core 10 there will be nine tooling blocks 40. One
tooling block is fitted with bottom dowel pins mating with holes 38 for
placement stationary in the center of the tool base 32. Four of the tooling
5 blocks 40 are corner blocks (one shown in position) having bottom slide-
tabs 406 oriented along the diagonal as shown in FIG. 8. The remaining
four tooling blocks are side blocks, for example block 40' in FIG. 5, having
bottom slide-tabs oriented along the center of the blocks. The ends of the
bottom slide-tabs are rounded to prevent the ends from being caught up on
10 the sides of slots 34, 36.
The bottom slide-tabs for the side blocks and the corner blocks, for
example block 40, are shorter in length than the corresponding slots 34, 36
in the tooling base 32, and they also are slightly narrower than the slots 34,
36. This allows the side blocks and corner blocks to move back and forth
15 when inserted in the tool base 32 slots 34, 36. The tool base 32 slots 34,
, 36 are oriented toward the center and are of a sufficient length to allow all
the movable corner and side blocks to slide with minimal friction toward the
stationary center, for example block 40" in FIG. 5, when their bottom tabs
are inserted into the respective tool base 32 slots 34, 36.
20 The thickness of the ribs 12 (in FIG. 1) are determined by the
thickness of the prepreg material, the autoclave pressure during curing, and
the slot geometry of the curing jig 30. By positioning appropriate tooling
stops at the center end of the slots 34, 36 to limit the travel of the blocks,
a fixed rib 12 thickness can be assured, regardless of autoclave pressure.
25 The side and corner tooling blocks (i.e., blocks 40, 40') must be able to
approach the center block (i.e., block 40") to a distance not exceeding the
required maximum core rib 12 thickness. For greater process flexibility, the
slot dimensioning should enable the side and corner blocks to slide far
enough toward the center to just touch the center block before prepreg
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strips 50 (see FIG. 5) are added.
Four caul plates 45, as shown in FIG. 5 are cut strips of sheet metal
placed along the outer edges of the tooling block assembly during
processing. The specific dimensions of the tooling are arbitrary and depend
upon the required dimensions of the finished eggcrate rib-core 10.
After processing, the tooling blocks 40, 40', 40" are removed, and
the top and bottom faces of the core 10 are conventionally trimmed. The
resulting structure represents a composite co-cure modular eggcrate rib-
core. Two laminate face sheets can now be secondarily bonded to the core
10 by any method used in the prior art.
In order to obtain quasi-isotropic ribs 12 from the tooling block 40,
40', 40" assembly as shown in FIG. 5, the layup must be arranged to take
into account the fact that layups on any two adjacent tooling blocks are
flipped around the vertical, so 45 degree ply orientations change signs. If
each tooling block 40, 40', 40" were wrapped with a 4-ply layup with
orientations [0,45,90,-45], the finished laminate between each pair of
tooling blocks would, accordingly, be oriented [0,45,90,-45,45,90,-45,0],
and the [-45, 45] degree orientations in the central layers would cause an
undesirable 90 degree transition between these two layers. This can be
avoided by instead wrapping all tooling blocks 40, 40', 40" and the
perimeter of the tooling block assembly with 4-ply layups with orientations
[45,0,-45,90], and arranging the 45 degree layers in contact with the
tooling block surfaces. All center plys would then have the same 90 degree
orientation.
Alternatively, the central tooling block 40" and all four corner blocks
40 can be wrapped with 4-ply layup strips oriented [0,45,90,-45], while the
side tooling blocks 40' are wrapped with 4-ply layup strips oriented [0,-
45,90,45]. After assembly, all the internal ribs would then have [0,45,90,-
45,-45,90,45,0] layer orientations, forming the same balanced, symmetric,
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8-ply laminate as in a conventional rib layup. If the outer strips 50' are also
4-ply strips with layup orientations [0,-45,90,45], the ribs at the outer
perimeter of the corner blocks 40 will also have the desired [0,45,90,-45,-
45,90,45,0] layup. The four outer ribs at the side blocks 40' will, however,
5 have [45,-45] orientations in their center layers. In most applications this
is acceptable, because any effects of partial residual stresses imparted by
the unsymmetric alternating ribs, which comprise the perimeter, would be
isolated at the edges. This is expected to contribute a negligible effect on
the overall dimensional stability of the sandwich structure. Alternatively,
10 the design can allow the perimeter ribs at the corner blocks 40 and the side
block ribs 40' to be removed following cure. The inclusion of outer ribs to
form a core perimeter is a necessary result of the process, but is not
necessary to tile design intent for dimensional stability and strength.
The present invention may be extended from cores 10 to composite
15 sandwich structures. FIGS. 11 - 13 illustrate a preferred method for making
, a core 10 with a single co-cured face sheet 60. The second core face is
prepared for secondary bonding of a second face sheet by conventional
methods.
This method uses the same tooling blocks 40, 40', 40" and curing
20 jig 30 as described with reference to FIGS. 5 and 8, but in this case the
laminate strips 50" are cut to a width in excess of the height of the tooling
blocks. The excess width of the laminate strips 50" is arranged to extend
beyond the top of each tooling block 40, 40', 40" as illustrated in FIG. 11,
and is cut to form core tabs 53. The core tabs 53 are pressed down to lie
25 flat on top of each tooling block, as illustrated in FIG. 12.
All the nine wrapped tooling blocks 40, 40', 40" are placed on the
curing jig 30, and strips of prepreg material 50' with width equal to the
height of the tooling blocks 40, 40', 40" plus tabs 53 are added to the
periphery of the tooling block assembly. The tooling block assembly is then
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compacted and debulked as previously described. A pre-cured (or uncured)
face sheet 60 may now be positioned over the core tab 53 surfaces, and
caul plates 45 and 46 added. The entire assembly is finally bagged for
autoclave curing.
5 The ribs 12 between and around the tooling blocks 40, 40', 40" will
be co-cured as previously described, and at the same time the prepreg
laminate 60 on top of the tooling block assembly will co-cure with the core
tabs 53 on top of the tooling blocks to form an integral co-cured face panel
for the sandwich structure. In the process, autoclave pressure acts to
10 consolidate the core ribs 12, through the tooling blocks 40, 40', 40". At
the same time the autoclave pressure acts to apply equal pressure to the
face sheet 60 and core tab 53 interface. The core tabs 53 provide a large
contact area for" the face panel 60. After curing, the tooling blocks 40, 40',
40" are removed from the sandwich structure.
15 In some cases it is preferable to cure the core 10 with core tabs 53
• on top of the tooling blocks 40, 40', 40" separately, and add the face panel
60 in a separate process. After curing of the core 10 with core tabs 53, the
pre-cured (or uncured) face panel 60 may be positioned over adhesive strips
placed on the core tab 53 surfaces. The complete assembly is now
20 prepared for conventional autoclave cure, with equal pressure applied to the
interface between the face sheet 60 and the adhesive covered core tabs 53.
The core tabs 53 provide a large contact area for the adhesive for the face
panel 60, and the adhesive film provides precision mounting of the face
panel 60 on the core 10.
25 The method described above for providing core tabs 53 on top of the
tooling blocks can also be extended to provide core tabs 53 on both the top
and the bottom of tooling blocks. In this case, the width of the iaminate
strip 50" is evenly distributed on both sides of the tooling block (40, 40',
40") as the laminate strip 50" is wrapped around it. The corners of the
Page 19
LAR 14898-1 -17- PATENT APPLICATION
excess strip width on both sides of the tooling block are then cut to enable
the excess to fold onto both the top and the bottom surfaces of the tooling
block. This produces four folded core tabs 53 on the top and four similar
folded core tabs 53 on the bottom of each tooling block 40, 40', 40".
After curing of the core 10 with core tabs 53 (with or without co-curing a
top face panel), cured core tabs 53 will also be formed between the bottom
surfaces of the tooling blocks 40, 40', 40" and the base plate 32 of the
curing jig 30.
When core tabs 53 have been formed on the bottom side of the core
for secondary bonding of a bottom face sheet, the tooling blocks 40, 40',
40" must be made from wash-out (eutectic salt) or melt-out (low
temperature melt alloy) materials, or any other material that can be removed
from the core ir_terior after the cure process.
The process described above may also be extended to co-curing of
two face sheets, thereby eliminating the secondary bonding step for the
second face sheet. The layup and processing is similar to that discussed
above with reference to FIGS. 11 - 13, except that the tooling blocks 40,
40', 40" will remain inside the completed sandwich structure. For purposes
of weight savings, it is preferable that the tooling block 40 material be made
from commercially available structural foam. The laminate layup and top
and bottom core tabs 53 are formed as described with reference to FIGS.
11 - 13above.
Since both sides of the core assembly are co-cured to a laminate face
sheets, the tooling blocks 40, 40', 40" have no slot-tabs and are not placed
on a curing jig 30. Instead, to facilitate assembly and alignment, the tooling
blocks are placed, one at a time, in an assembly frame (not shown). The
assembly frame should be adjustable and calibrated in both x and y
directions. It may be adjusted along vernier scales to fix the alignment and
the required length and width of the tooling block assembly.
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Page 20
LAR 14898-1 -18- PATENT APPLICATION
After the core is assembled and aligned, the frame is released. The
tack of the material holds the assembly in place. Debulking procedures are
then followed using shrink tape, and the side laminate strips are positioned,
followed by a second debulk. The assembly frame is again positioned
5 around the assembly for subsequent transport. Enough pressure can be
applied to the tool block assembly from the assembly frame walls to enable
the assembly to be picked up and maneuvered without movement occurring
among the tooling blocks.
Adhesive strips are positioned along the core tabs (53), and the first
10 face sheet is placed on the core/tooling block assembly. The assembly is
then turned over and the process is repeated on the other side for the
second face sheet. The assembly is then prepared for conventional
autoclave cure. "In the process, equal pressure will be applied to the top and
bottom face-sheetadhesive-core-tab interfaces as well as to the core walls
15 through the structural foam tooling blocks. Here, the matrix resin itself acts
, as a lubricant to enable the tooling blocks 40, 40', 40" surfaces to slide on
the face sheet surfaces toward the center, facilitating rib compaction. The
completed structure comprises the core, two face sheets, and the
incorporated foam tooling
20 blocks.
An alternative method for making a core with one co-cured face sheet
will be described with reference to FIGS. 14 - 17. The same tooling blocks
40, 40', 40" and curing jig 30 as described with reference to FIGS. 5, 8,
and 13 are used, but the tooling blocks 40, 40', 40" are not wrapped
25 horizontally with prepreg strips in this case. Instead, two laminate strips 54
and 55 are draped crosswise over the top and down opposite sides of each
tooling block 40, 40', 40". The wrapped tooling blocks are placed on the
curing jig 30 with side strips 50' of prepreg added, and the assembly is
debulked. After co-curing and removal of the tooling blocks (40, 40', 40"),
Page 21
LAR 14898-1 -19- PATENT APPLICATION
ribs 12, as in FIG. 1, are formed between pairs of voids 14, 14', 14", and
a face sheet is formed on top of each void 14, 14', 14'. The face sheet
may have shallow valleys above the ribs 12, but a strong and rigid
sandwich structure has been formed in one curing step.
5 There will be no continuous fibers extending around the corners
between the voids 14, 14', 14" but the corners 13 will be filled with matrix
material, and the ribs 12 provide vertical and lateral stability to the
sandwich structure. If extra fiber reinforcement is desired at corners 13,
narrow strips of prepreg material may be arranged to cover the gaps
10 between the prepreg strips 54, 55 before assembly of the wrapped tooling
blocks.
FIG. 16 is a diagram showing the formulation of a quasi-isotropic
structure with near zero CTE. For purposes of demonstration an 8-ply layup
is used for the finished ribs, the top panel, and the edges of the finished
15 structure. A representative structure is obtained by cutting P75/934 tape
, into single-ply strips with width equal to the width of the tooling blocks 40
and length equal to that of the sum of the top and two sides of the tooling
blocks 40. Each strip has a specific fiber orientation, the total number of
each is sufficient to produce one-half of the rib wall layup as [0,-45,90,45]
20 ("A), and the other half as [-45,0,45,90] ("B"). One A-strip is draped over
the top of each tooling block 40, 40' 40" and down along two opposite
sides, and one B-strip is draped crosswise over the A-strip. This is followed
in sequence by another A-strip, which is again crossed by the next
sequential B-strip. The layup continues until the completed A and B layups
25 are achieved. All the tooling blocks 40, 40', 40" are then placed on the
curing jig 30 with all A-strips parallel and all B-strips parallel as indicated in
FIG. 16. Four additional strips are cut and fitted along the circumference
of the tooling block assembly.
FIG. 17 is a diagram showing the preferred fiber orientations for the
Page 22
LAR 1
5
4898-1 -20- PATENT APPLICATION
quasi-isotropic structure shown in FIG. 16. Note that the fiber orientation
in strips 54, 55 is referenced to the length dimension of the strips, but the
orientations in FIG. 17 are referenced to the assembly, as shown. The final
fiber orientations for all panels and ribs will be as indicated in FIG. 17,
which means that all panels meet the strictest standard for quasi-isotropic
and zero CTE lay-up. After debulking and curing, and removal of the tooling
blocks, a true quasi-isotropic sandwich structure with a discrete rib-core and
one integral face panel is obtained.
Alternatively, a window may be cut in the top plys on each tooling
10 block 40, 40', 40" so frame-like tabs are formed on top of each tooling
block. A separate face panel with the same lay-up orientations as in the
tabs can then be bonded to the tabs, either by co-curing, or by bonding
with adhesive film strips.
The composite modular structures and processes described above
15 with reference to eggcrate rib-core designs may be extended to other
, application in which ribs are separated by voids.
FIGS. 9 and 10 are top views of honeycomb cores. FIG. 9 shows a
near circular core 10', and FIG. 10 shows a rectangular core 10" In both
cases, the central part of the cores (10', 10") are built from laminate walls
20 defining hexagonal voids 140. The outer parts of the cores 10', 10" are
filled out by laminate walls defining triangular voids 142, 144, 146 or
trapezoid voids 148. All the voids fit together like tiles, so the walls of
adjoining voids can form ribs 12.
The method for making cores 10', 10" is exactly the same as
25 described above for an eggcrate rib-core 10, except for the cross sections
of the tooling blocks and obvious alterations in curing jigs or assembly
frames. Tabs for bonding to face sheets may also be added as integral
extensions of the ribs, as described above with reference to FIGS. 11 - 13.
One or two face sheets may be co-cured with such tabs, exactly as
Page 23
LAR 14898-1 -21- PATENT APPLICATION
described above for eggcrate rib-cores with co-cured face sheets.
Structures composed of other fiber/resin materials and having other
unit cell geometries or more complex shapes may be fabricated using the
same principles, using corresponding tooling block designs. The principle
of the invention is to form ribs by joining walls around tooling blocks, which
replace the voids during a cocuring process and are removed after curing.
This principle can be extended to any core design based on voids fitting
together like tiles, and the ribs need not even be straight. For some
complex shapes, washout or meltout tooling blocks may be required to
enable removal of the tooling blocks after processing.
The co-cure modular eggcrate rib-core according to the invention has
significantly improved strength, stiffness, and overall property uniformity
compared to sirfiilar structures produced by prior art methods. Mechanical
load distribution is more uniform, as are such physical properties as thermal
expansion and conductivity.
There are no finite joint locations within a_ core according to the
invention in which significant interfacial tension loads are carried by or
transferred to a weak adhesive medium. Loads are carried by the superior
tensile, compressive, and interlaminar shear properties of the laminate itself
and transferred throughout the structure. This distribution serves to
significantly reduce the load stresses, greatly improving the structure's
strength attributes. In addition, unlike the case of adhesive failure in which
load carrying ability is abruptly halted, the failure mode within the fiber/resin
laminate occurs very gradually through interlaminar delamination, while still
maintaining load carrying capability, because load energy is absorbed
through crack initiation and extended growth. This is not poss_'ble in the
conventional rib-core approach since the fibers are cut to create slots at the
joints, thus relying on unreinforced adhesive to transfer all loads.
Since only fiber/resin material is used for the structure, its properties
5
10
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25
Page 24
LAR 14898-1 -22- PATENT APPLICATION
are more uniform and can be more reliably predicted from design. Outside
the basic fiber/resin material selected by design, there are no other materials
to cause CTE mismatch and thermal stresses. Physical transport properties
are more uniform within a uniform medium.
5 Secondarily bonded reinforcement materials, such as buttclips, used
by prior art to improve the joint strength, are not required.
For the same property design requirements, the co-cure modular core
obviously has less weight than the prior art rib-core design, because there
are no adhesives or reinforcement clips.
10 The extension of this process to include co-cure of the face sheets
onto tabs integral with the core (FIGS. 13, 15) significantly improves face-
sheet-to-core interfacial strength and load distribution. This occurs since
the core tabs formed in this process increase the effective bonding area at
the face sheet/core interface. Compared to the secondary bonding of the
15 face sheets to only the edge-width of the core laminate, there are strength
, advantages in bonding to a wider core area. In the case where an uncured
face sheet laminate is co-cured with the core, no adhesive would be
required. The absence of adhesive would promote greater uniformity in
physical and mechanical properties (CTE, strength and stiffness) and would
20 reduce the overall weight of the sandwich structure.
The present invention provides many process advantages over prior
art processes for making composite cores and sandwich structures.
The present invention provides a co-cure process in which the
fabrication of the composite core laminate and the assembly of the core
25 structure are accomplished in unison. No secondary bonding using adhesive
is required for the manufacture of a core, and no labor-intensive or costly
procedures are required. All procedures facilitate easy working access for
assembly. No bonding tools are needed for alignment, since alignment is
inherent in and provided by the co-cure tooling.
Page 25
LAR 14898-1 -23- PATENT APPLICATION
5
10
15
2O
25
No precision machining is required for assembly. There are no
precision machined slots or ribs. There is no rework required, to ensure
correct slot depth and alignment. The fabrication procedures are highly
simplified, so the labor experience base may be quickly established or
transitioned. The manufacturing process is highly controllable, so high
quality can be easily established and maintained. Only a final trim of the
core surfaces after fabrication is needed. This type of machining is
conventionally applied to the surfaces of honeycomb cores. The one-time
machining of the overall core surfaces is less prone to outside error
tolerance compared to the repetitive machining of multiple ribs, which,
when assembled together, would comprise the overall core surfaces
according to the prior art.
No adhesive procurement, test qualification, inventory or storage is
required. This again saves time, labor, and cost. Error tolerance stack-up
is less than in the prior art approach. Fewer procedural steps means fewer
procedural errors. Prior art problems with slot sizes, variation in adhesive
composition and application, rib alignment variation, and adhesive residual
stresses, are all avoided by the invention. The steps in the co-cure modular
assembly according to the invention are much less prone to errors that
would significantly affect the final properties than the prior art rib-core
approach.
The process according to the invention is also more conducive to
scale-up. For a larger structure the same tooling blocks and procedures are
applied, except in more quantity. In contrast, the prior art rib-core approach
requires extended machine precision for more ribs and slots, more
application of paste adhesive, more intricate alignment mechanisms and
verifications, all contributing to a greater error tolerance stack-up.
Manufacturing lead-time is also significantly reduced. Again, fewer
steps, less likelihood for error and rework, co-cure fabrication and assembly,
Page 26
LAR 14898-1 -24- PATENT APPLICATION
less labor expertise required, all contribute to lessen the manufacturing lead-
time.
Overall labor and material costs are significantly reduced.
The present invention also provides many benefits in the manufacture
of co-cured face sheet/modular core structures. Except for minor face sheet
edge-trim and resin flash removal, no machining is required at all.
Secondary bonding of face sheets is reduced or eliminated. Lead-time,
labor, re-work, materials, and total cost are all significantly reduced. Ready-
to-use film adhesive rather than component-mix paste adhesive may be
used. Film adhesive may be easily cut into strips and selectively placed on
the core-tabs, so no unnecessary adhesive is applied. Film adhesives can
not be used to bond face sheets to the edge-width of the core. A tedious,
time-consuming'application of paste adhesive to the core edges of both core
sides is thus avoided.
Numerous modifications and adaptations of the present invention will
be apparent to those skilled in the art. Thus, the following claims are
intended to cover all such modifications and adaptations which fall within
the true spirit and scope of the present invention.
5
10
15
Page 27
LAR 14898-1 PATENT APPLICATION
COMPOSITE SANDWICH STRUCTUREAND
METHOD FOR MAKING SAME
5
10
15
Abstract nf the, Disclosure
A core for a sandwich structures which has multi-ply laminate ribs
separated by voids, is made as an integral unit in one single curing step.
Tooling blocks corresponding to the voids are first wrapped by strips of
prepreg layup equal to one half of each rib laminate so a continuous wall of
prepreg material is formed around the tooling blocks. The wrapped tooling
blocks are next pressed together laterally, like tiles, so adjoining walls from
two tooling blocks are joined. The assembly is then cured by conventional
methods, and afterwards the tooling blocks are removed so voids are
formed. The ribs can be provided with integral tabs forming bonding areas
for face sheets, and face sheets may be co-cured with the core ribs. The
new core design is suitable for discrete ribcores used in space telescopes
and reflector panels, where quasiisotropic properties and zero coefficient of
thermal expansion are required.
Page 28
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Page 31
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